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Systems for Handling Heavy Subsea Loads

5/26/2008

MacGREGOR’s Offshore division delivers complete equipment packages for subsea intervention vessels. Its Hydramarine package designed for a new inspection, maintenance and repair (IMR) vessel Edda Fauna includes a 60-tonne module handling system. The challenging nature of subsea operations means that load handling cannot always be undertaken by cranes, and in these circumstances the only alternative are module-handling systems of the type pioneered by Hydramarine. The module handling system’s great advantage is that it can undertake the kind of guided load handling that a crane cannot, thus eliminating many of the risks of non-guided lifts.

The system’s load carrying structures are part of the vessel structure and even skid beams are fully integrated into the vessel’s deck. The advantages of this solution compared with a ‘modularised add on’ system are a better working environment, more space and better overall safety under all operation conditions. The entire tower system is in this case located inside a large heated work hangar and the system is well suited for arctic operations.

Edda Fauna can handle modules of up to 60 tons with dimensions of 6m x 6m x 8m to a maximum depth of 2,000m in sea states with significant wave heights of up to 5m. The system consists of a rail skid arrangement on the main deck for horizontal transportation of loads, and a cursor system, liftline and guideline winches for vertical transportation through the ship’s moonpools. Horizontal movement of modules is achieved by sliding pallets which are moved by hydraulic tractors or ‘pushers.’ The module handling system has four different types of pallets with their own tractors: a 60 ton pallet, two different sizes of 30-ton pallets, and a 3-ton/10m crane pallet.

The 7.2m x 7.2m main moonpool is amidships. It has hatches, guiding cursor and active heave-compensated winch system for vertical movements. The main moonpool is also fitted with four active heave-compensated guideline winches to guide the modules from deck level, and the system has a separate active heave-compensated liftline winch. The two smaller 4.8m x 4.8m moonpools are in the ROV hangar, and can each handle a work-class ROV. To the port side of the hangar an A-frame is located for vertical handling of an observation-class ROV.

The Hydramarine electric driven, active heave compensated ROV umbilical winches and Guide Line Winches greatly improve the performance in many ways. First of all, the simplicity and reduced number of critical components increase the overall system reliability. Accuracy in position and force control are at highest level and adjustments possible with exceptional performance. Comfort onboard vessel is improved as the electrical winches have very low noise and vibration levels. There are no requirements for high performance HPU system and advanced critical servo valves, which often are a source for vibration, noise and technical problems.